1. Field of the Invention
The present invention relates to a suspension system for a vehicle and, more particularly, to the suspension system which can enhance driving safety and marginal cornering performance by optimally controlling the camber and tread changes as well as roll center in accordance with vehicle's driving conditions.
2. Description of Related Art
In general, the primary functions of a suspension system are to (A) provide vertical compliance so wheels can follow the uneven road, isolating a vehicle body from roughness in the road, (B) maintain the wheels in the proper steer and camber attitudes to the road surface, and (C) react to the control forces produced by the tires-longitudinal (acceleration and braking) force, lateral(cornering) forces, and braking and driving torques, (D) resist roll, yaw and pitch of the vehicle body.
The suspension system is structurally characterized as solid axle and independent suspension types. The suspension system of the present invention concerns the independent suspension system. There are MacPherson strut type and Wishbone type suspension systems in the independent suspension systems. And in recent years, a multi-link type independent suspension system has been popular.
FIG. 6A illustrates a general MacPherson strut type suspension system which comprises a strut assembly 155 formed by uniting a shock absorber 151 and a spring 152 and having a lower end fixed to a wheel carrier 153 rotatably supporting a wheel 150 and an upper end supportably connected to a vehicle body 154; and a lower control arm 156 connecting a lower end of the wheel carrier 153 to a lower side of the vehicle body 154.
The range of a wheel's up-and-down movement in this suspension system is determined by the lower control arm 156 during a wheel's up-and-down vibration. The vibration is absorbed by an elastic bushing formed on an inner end of the lower control arm. Shock from a road surface is absorbed by the shock absorber 151 and spring 152 of the strut assembly 155.
However, when considering operation steps of the above described suspension system, if the wheel 150 vibrates upward and downward in accordance with driving conditions, the wheel 150 moves upward and downward in a state where the length of the lower control arm 156 is fixed.
Accordingly, the wheel 150 moves upward and downward along a tracing movement M which is formed by the length of lower control arm 156. A camber and a tread are changed according to the wheel's up-and-down movement and has a great effect on riding comfort, cornering performance, and driving safety of the vehicle.
That is, since the outer end of the lower control arm 156 is designed to be lower than the wheel center, when the wheel moves along the tracing movement M, in a normal ascending section, the lower side of the wheel is pushed outward, that is, to the wheel side, along a curvature of the tracing movement M such that the camber is changed to negative(-).
However, in a section over the normal ascending section, the lower control arm 156 ascends and pulls the lower side of the wheel to the vehicle body side, whereby the camber is changed again to positive(+) (See broken line of FIG. 4A).
The change of the camber as described above is illustrated in broken line of FIG. 4A. Further a tread is changed as illustrated in broken line of FIG. 4B by the change of the camber described above. Such changes of the camber and tread have a bad effect on the cornering performance and the straight-ahead driving safety.
FIG. 6B illustrates a general Wishbone type suspension system, including upper and lower control arms 61 and 62, a steering knuckle 63, a spring assembly 65 including a shock absorber 64, and ball joints 66 and 67 respectively connecting the upper and lower control arms 61 and 62 to the steering knuckle 63.
This type of suspension system has a problem that when the wheel 69 moves upward and downward, since the upper control arm 61 has a small curvature tracing movement and the lower control arm 62 has a large curvature tracing movement, the tread and camber are severely changed(See dot-dash line of FIGS. 4A and 4B).
Further, as shown in broken line of FIG. 5, in the Wishbone and Macpherson type suspension systems, when the vehicle is cornering, the more wheel on the outer side of turn to receive the lateral force, the more camber is changed to the positive(+), thereby deteriorating the marginal cornering performance.